Differential measurements allow correction of fixed-pattern noise errors in digital imaging arrays which use time-to-threshold A/D conversion techniques. Two time-to-threshold measurements are made with the same sensor and threshold-detecting circuitry. The measurements are made in quick succession so that the amount of incident energy is substantially unchanged. However, the two measurements use differing initial sensor output levels or threshold levels. The difference between the two measurements then reflects the time required for each sensor output signal to change by an amount equal to the difference between the initial sensor output values or the threshold values. Repeatable noise terms are cancelled in the computed difference measurement.
Legal claims defining the scope of protection, as filed with the USPTO.
1. A method for differential elapsed-time analog-to-digital conversion, comprising: a. initializing an output of a first sensor to a first initial output level b. allowing said output of said first sensor to change from said first initial output level in response to incident energy c. measuring a first elapsed time required for said output of said first sensor to change from said first initial output level to a first threshold level d. initializing said output of said first sensor to a second initial output level e. allowing said output of said first sensor to change from said second initial output level in response to incident energy f. measuring a second elapsed time required for said output of said first sensor to change from said second initial output level to a second threshold level g. combining said first elapsed time and said second elapsed time whereby in conditions of substantially constant incident energy said first elapsed time and said second elapsed time are functions of said first initial output level, said first threshold level, said second initial output level, and said second threshold level.
2. The method of claim 1 in which said first threshold level is substantially the same as said second threshold level.
3. The method of claim 2 in which said first initial output level is different from said second initial output level.
4. The method of claim 3 in which said combining comprises computing a difference time between said first elapsed time and said second elapsed time, whereby said difference time does not require precise knowledge of said first threshold level and said second threshold level.
5. The method of claim 1 in which said first initial output level is substantially the same as said second initial output level.
6. The method of claim 5 in which said first threshold level is not the same as said second threshold level.
7. The method of claim 6 in which said combining comprises computing a difference time between said first elapsed time and said second elapsed time, whereby said difference time does not require precise knowledge of said first initial output level and said second initial output level.
8. The method of claim 1 further including: a. initializing an output of a second sensor to a third initial output level b. allowing said output of said second sensor to change from said third initial output level in response to incident energy c. measuring a third elapsed time required for said output of said second sensor to change from said third initial output level to a third threshold level d. initializing said output of said second sensor to a fourth initial output level e. allowing said output of said second sensor to change from said fourth initial output level in response to incident energy f. measuring a fourth elapsed time required for said output of said second sensor to change from said second initial output level to a second threshold level g. combining said third elapsed time and said fourth elapsed time.
9. The method of claim 8 in which: a. said third initial output level is substantially the same as said fourth initial output level b. said third initial output level is substantially the same as said first initial output level.
10. The method of claim 8 in which: a. said allowing of said change from said first initial output level is substantially simultaneous with said allowing of said change from said third initial output level b. said allowing of said change from said second initial output level is substantially simultaneous with said allowing of said change from said fourth output level whereby elapsed time measurements for two sensors are made substantially simultaneously.
11. A machine used for differential elapsed-time analog-to-digital conversion, comprising: a. means for initializing an output of a first sensor to a first initial output level b. means for allowing said output of said first sensor to change from said first initial output level in response to incident energy c. means for measuring a first elapsed time required for said output of said first sensor to change from said first initial output level to a first threshold level d. means for initializing said output of said first sensor to a second initial output level e. means for allowing said output of said first sensor to change from said second initial output level in response to incident energy f. means for measuring a second elapsed time required for said output of said first sensor to change from said second initial output level to a second threshold level g. means for combining said first elapsed time and said second elapsed time whereby in conditions of substantially constant incident energy said first elapsed time and said second elapsed time are functions of said first initial output level, said first threshold level, said second initial output level, and said second threshold level.
12. The machine of claim 11 in which said first threshold level is substantially the same as said second threshold level.
13. The machine of claim 12 in which said first initial output level is different from said second initial output level.
14. The machine of claim 13 in which said means for said combining comprises means for computing a difference time between said first elapsed time and said second elapsed time, whereby said difference time does not require precise knowledge of said first threshold level and said second threshold level.
15. The machine of claim 11 in which said first initial output level is substantially the same as said second initial output level.
16. The machine of claim 15 in which said first threshold level is not the same as said second threshold level.
17. The machine of claim 16 in which said means for said combining comprises means for computing a difference time between said first elapsed time and said second elapsed time, whereby said difference time does not require precise knowledge of said first initial output level and said second initial output level.
18. The machine of claim 11 further including: a. means for initializing an output of a second sensor to a third initial output level b. means for allowing said output of said second sensor to change from said third initial output level in response to incident energy c. means for measuring a third elapsed time required for said output of said second sensor to change from said third initial output level to a third threshold level d. means for initializing said output of said second sensor to a fourth initial output level e. means for allowing said output of said second sensor to change from said fourth initial output level in response to incident energy f. means for measuring a fourth elapsed time required for said output of said second sensor to change from said second initial output level to a second threshold level g. means for combining said third elapsed time and said fourth elapsed time.
19. The machine of claim 18 in which: a. said third initial output level is substantially the same as said fourth initial output level b. said third initial output level is substantially the same as said first initial output level.
20. The machine of claim 18 in which: a. said means for said allowing of said change from said first initial output level is substantially simultaneous with said allowing of said change from said third initial output level b. said means for said allowing of said change from said second initial output level is substantially simultaneous with said allowing of said change from said fourth output level whereby elapsed time measurements for two sensors are made substantially simultaneously.
21. A method for differential elapsed-time analog-to-digital conversion in a digital imaging system, comprising: a. initializing an output of a first sensor to a first initial output level b. allowing said output of said first sensor to change from said first initial output level in response to incident energy c. measuring a first elapsed time required for said output of said first sensor to change from said first initial output level to a first threshold level d. initializing said output of said first sensor to a second initial output level e. allowing said output of said first sensor to change from said second initial output level in response to incident energy f. measuring a second elapsed time required for said output of said first sensor to change from said second initial output level to a second threshold level whereby in conditions of substantially constant incident energy said first elapsed time and said second elapsed time are functions of said first initial output level, said first threshold level, said second initial output level, and said second threshold level.
22. A machine used for differential elapsed-time analog-to-digital conversion in a digital imaging system, comprising: a. means for initializing an output of a first sensor to a first initial output level b. means for allowing said output of said first sensor to change from said first initial output level in response to incident energy c. means for measuring a first elapsed time required for said output of said first sensor to change from said first initial output level to a first threshold level d. means for initializing said output of said first sensor to a second initial output level e. means for allowing said output of said first sensor to change from said second initial output level in response to incident energy f. means for measuring a second elapsed time required for said output of said first sensor to change from said second initial output level to a second threshold level whereby in conditions of substantially constant incident energy said first elapsed time and said second elapsed time are functions of said first initial output level, said first threshold level, said second initial output level, and said second threshold level.
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March 4, 2004
March 29, 2005
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